Spectral measurements have proved that activation of phenylalanine hydroxylase changes its conformation. A 4 Alpha-carbinolamine tetrahydropterin has been shown to be an intermediate in the hydroxylation reaction. Phenylalanine hydroxylase stimulator protein has been shown to be a 4 Alpha-carbinolamine dehydratase. Glucagon stimulates phenylalanine hydroxylase activity in the whole rat.